[Taler] impossibility of payment replay with the current spec

[Florian Dold]

Hi all,

a while ago we added a mechanism to the wallet so that payments are
executed from a cooperative merchant page (which the merchant has to
provide in addition to the contract), so that the POST request to /pay
is from the same origin as the earlier parts of the purchase, and thus
session cookies will be delivered.

I think this approach is wrong, and the merchant's /pay should never
rely on session cookies or any unspecified request parameters.

Instead, we must have a well-defined API for /pay across merchants,
which only takes the contract and the deposit permissions.

This has two reasons:

1. It obsoletes the cooperative execution page, and makes the spec
clearer and simpler.  This is a minor reason though

2. More importantly, without a well-defined /pay API for merchants, it
is impossible to replay contracts in general.

Basically, if I have a contact and the deposit permissions, I should
*always* be able to submit that to the merchant, *independent* of any
session state that the merchant has.

Right now, payment replay for the demo shop is technically impossible
due to the way the merchant is implemented (even though it follows the
spec), no matter what the wallet does.  The /pay page requires a session
that stores the contract hash we're about to pay for, and there is no
general way for the wallet to achieve this state.

Now, we could add another kludge and require some "session reinstatement
page" (which is again a cooperative page from the merchant that gives us
the session that's required to pay), but I don't see any good arguments
for that, in fact it seems rather insane.

Any thoughts?

- Florian